It simply lets you track a particular thread and you get notification of new posts. Pick a thread and try it out if you want, since you can always unsubscribe if you decide you don't want it after all.

From MST3K's spoof of "Hercules Unchained"--heard as Roman medic soldiers carry off an unconscious Greek Hercules on a 1950's Army green canvas stretcher: "Hi, we're IX-I-I. Did somebody dial IX-I-I?"

It's worth noting that you can set your profile to several different subscribe options, the first of which simply adds an area to the front page of your UserCP; this area lists the threads you're subscribed to that've received new posts since you last visited. I find it very useful as I like to track threads, but not be emailed every time.